1. Field of the Invention
The present invention generally relates to low-pass filters, and more particularly, to a low-pass filter for use in an audio apparatus.
2. Description of the Related Art
Conventionally, a digital signal input to an audio apparatus is converted into an analog signal by a digital analog converter (hereinafter referred to as xe2x80x9cDACxe2x80x9d) in the apparatus. The resulting analog signal has its high frequency component attenuated by a low-pass filter (hereinafter referred to as xe2x80x9cLPFxe2x80x9d) in the audio apparatus and is then output from the apparatus.
FIG. 17 is a circuit diagram of the configuration of an LPF in a conventional audio apparatus.
Referring to FIG. 17, the LPF 200 includes resistance elements R201 and R202, capacitors C201 and C202, and an operational amplifier OP201. The resistance elements R201 and R202 are connected in series, and one end of the resistance element R202 is connected to the non-inverting input terminal of the operational amplifier OP201. An analog signal is input to the end of the resistance element R201 not connected to the resistance element R202. The capacitor C201 has its one end connected between the resistance elements R201 and R202 and its other end connected to the output terminal of the operational amplifier OP201. The capacitor C202 has its one end connected to the non-inverting input terminal of the operational amplifier OP201 and its other end connected to a ground potential node 201. The inverting input terminal of the operational amplifier OP201 is connected to the output terminal.
In order to improve the sound quality of the analog signal output from the audio apparatus, the LPF must sufficiently remove the high frequency component from the analog signal. This is because the high frequency component can be a cause of degradation of the sound quality.
FIG. 18 is a waveform chart of the analog signal output from the LPF 200. As shown in FIG. 18, the LPF 200 cannot completely remove the high frequency component from the analog signal.
In order to improve the sound quality, the high frequency component must be more thoroughly removed.
Japanese Patent No. 3,134,403 discloses the invention directed to improvement in the sound quality. According to the invention disclosed by the patent document, the waveform of the output signal is smoothed in order to improve the sound quality.
According to the invention disclosed by the document, however, the high frequency component cannot fully be removed. Therefore, the high frequency component that can be a cause of degradation of the sound quality must be more thoroughly removed.
It is an object of the present invention to provide an LPF having improved capability of removing the high frequency component.
A low-pass filter according to the invention attenuates a high frequency component of an input signal and outputs a low frequency component of the signal as an output signal. The low-pass filter includes differential operation circuit, voltage-current conversion circuit, and capacitance element. The differential operation circuit subtracts the output signal from the input signal and outputs a differential signal. The voltage-current conversion circuit converts voltage of the differential signal output from the differential operation circuit into current. The capacitance element accumulates charges in current received from the voltage-current conversion circuit, and generates the output signal.
In the low-pass filter according to the invention, the voltage-current conversion circuit and the capacitance element form a filter circuit, so that the high frequency component can be significantly attenuated. The differential operation circuit produces the difference between the input signal and the output signal and adds the difference to the output signal as required. Therefore, a necessary frequency component is not removed. As a result, a high sound quality can be secured.
The low-pass filter according to the invention preferably further includes first filter circuit. The first filter circuit attenuates a high frequency component of the differential signal output from the differential operation circuit and outputs a low frequency component of the differential signal to be supplied to the voltage-current conversion circuit.
The low-pass filter according to the invention includes the filter circuit after the differential operation circuit and can therefore have improved capability of removing the high frequency component. In addition, the necessary frequency band is not attenuated, because the quality factor can be raised and the attenuation in the passband gain in the frequency characteristic can be restrained.
The low-pass filter according to the invention preferably further includes second filter circuit. The second filter circuit attenuates a high frequency component of an input signal and outputs a low frequency component of the signal. The differential operation circuit subtracts the output signal from the low frequency component of the input signal output from the second filter circuit and outputs a differential signal.
In this way, the low-pass filter can have improved capability of removing the high frequency component, because the filter order can be increased and the slope characteristic can be steeper.
A low-pass filter according to invention attenuates a high frequency component of an input signal and outputs a low frequency component of the signal as an output signal. The low-pass filter includes differential operation circuit, sample and hold circuit, voltage-current conversion circuit, and capacitance element. The differential operation circuit subtracts the output signal from the input signal and outputs a differential signal. The sample and hold circuit samples and holds the differential signal output from the differential operation circuit. The voltage-current conversion circuit converts voltage output from the sample and hold circuit into current. The capacitance element accumulates charges in current received from the voltage-current conversion circuit and generates the output signal.
The low-pass filter according to the invention can sufficiently remove the high frequency component to be a cause of noise by the use of the sample and hold circuit. As a result, the high frequency component can sufficiently be removed, and the slope characteristic can be steep.
The sample and hold circuit preferably includes a plurality of sample and hold units and a switch circuit. The plurality of sample and hold units sample and hold the differential signal in different timings from each other. The switch circuit sequentially switches the outputs of the plurality of sample and hold units.
In this way, while one of the plurality of sample and hold units receives sampling instructions, the other sample and hold units do not receive any sampling instructions. As a result, when the sampling instruction period is shortened, the plurality of sample and hold units can carry out sampling, which improves the sampling accuracy. Since errors in sampling can be reduced and the sampling period can be shortened, the output waveform can be smooth.
A low-pass filter according to the invention includes first and second low-pass filter circuit. The first low-pass filter circuit attenuates a high frequency component of an input signal and outputs a low frequency component of the signal. The second low-pass filter circuit attenuates a high frequency component of the output signal of the first low-pass filter circuit and outputs a low frequency component of the signal. The first low-pass filter circuit includes first differential operation circuit, sample and hold circuit, first voltage-current conversion circuit, and first capacitance element. The first differential operation circuit subtracts the output signal of the first low-pass filter circuit from the input signal and outputs a differential signal. The sample and hold circuit samples and holds the differential signal output from the first differential operation circuit. The first voltage-current conversion circuit converts voltage output from the sample and hold circuit into current. The first capacitance element accumulates charges in current received from the first voltage-current conversion circuit and generates the output signal of the first low-pass filter circuit. The second low-pass filter circuit includes second differential operation circuit, second voltage-current conversion circuit, and second capacitance element. The second differential operation circuit subtracts the output signal of the second low-pass filter circuit from the output signal of the first low-pass filter circuit and outputs a differential signal. The second voltage-current conversion circuit converts voltage output from the second differential operation circuit into current. The second capacitance element accumulates charges in current received from the second voltage-current conversion circuit and generates the output signal of the second low-pass filter circuit.
The low-pass filter according to the invention includes the two low-pass filter circuit. The output signal of the first low-pass filter circuit is further removed of the high frequency component by the second low-pass filter circuit. The output signal of the first low-pass filter circuit can have a smoother waveform by the use of the second low-pass filter circuit.